Hey James,
Great question, and one that I've spent hours with myself! Quick disclaimer, I'm going to use the word Aiki here, there are many different understandings of this word, but I'm using it as I was taught by my teacher, and I believe his teacher before him.

The great thing about Aiki, is that it allows us to follow our attackers mind and movements. If you are using Aiki to interact with someone/thing, you are trying to find ways to match/understand their movement and intent. This is what allows ideal Aikido techniques to "use the attackers force against them". When a technique happens, onr that is due to direct use of Aiki it is because an attacker made a "miscue" and over committed in their attack. This miscue is what allows the Aikidoka to effortlessly project or control the attacker. This is why in Aikido practice it is important for uke to fully commit to an attack, because we are training to use our techniques in these situations.

Outside of the Dojo, or in sparring sessions inside of the Dojo, attackers will try hard not to over commit. If we are being "Aiki" with this situation, we will simply move to the advantageous position and await a miscue, or for the attack to stop. There is no "need" to use a technique on an attacker, we can let his own miscalculations spell his demise. As long as we can keep ourselves in the superior position, we are in no danger.

This is all very easy to say, but is difficult to do. Short answer, if you can blend with your partner, you can stay in a superior position, if the attacker makes an over committed attack in order to get to your position, technique will become available. Until that over committed attack comes, we try and keep ourselves in the superior position.

Anyways, these are my views, I hope you explore the question and get a satisfying answer of your own! Take care.